As used herein, the term "instrument" refers to a pen, pointing device, mouse or cursor, the position of which on a predetermined surface is to be determined. The surface may take a variety of forms familiar to those skilled in the art; however, the present system is particularly applicable to the utilization of a glass surface on a LCD display to create what is presently referred to as electronic paper. That is, the instrument is utilized to "write" on the glass surface which presents a LCD display to the user at the glass surface upon which he or she is writing. The pen or cursor may be utilized to radiate a signal downward onto the tablet and a plurality of orthogonally positioned grid conductors and a detection system are used within the tablet to ascertain the location of the source of the signals and thus the position of the instrument. The utilization of such instruments has usually entailed the use of an electric wire connection from the instrument to the tablet or the host computer; however, cordless instruments have recently become available and provide numerous advantages. If the cordless instrument is to be a source of signals for the determination of the instrument position on the tablet, then the instrument must contain a means for directing such signals onto the tablet. One technique that has been suggested in the prior art is the generation of signals within the tablet to be directed outwardly of the tablet and intercepted and redirected by the instrument back onto the tablet. While this type of system provides for the utilization of a cordless pen or instrument, the power required to transmit the signal to the pen for its subsequent retransmission to the tablet as well as the complexity of the system are distinct disadvantages. The power requirements are particularly disadvantageous when such system is to be utilized in portable applications demanding minimum power drain. Another technique that may be employed in the utilization of a cordless instrument or pen is the provision of a source of power within the instrument itself. A significant problem that is inherent in the latter cordless instrument designs is the need for a power supply such as a battery to be carried within the instrument itself. Obviously, if the instrument is a pen, the power supply must be physically small and the system within the pen must not require more power than can reasonably be expected for a significant lifetime of the battery supply.
The present invention utilizes a self-powered cordless pen to generate an electromagnetic signal directed into the grid of the tablet. The use of a self-powered cordless pen presents many substantial advantages over the above mentioned bi-directional signal system. That is, the instrument of the present invention permits greater signal levels to be sensed in the tablet, permits unidirectional transmission and thus significantly reduces the complexity required for the positional determination of the instrument. Systems incorporating cordless pen designs for use with magnetic tablets have in the past required coherency in the detection of grid signals emanating from the pen. That is, the prior art has required a coherent reference so that the phase of the signals being sensed on the grid lines in the tablet can be detected. This coherent detection is necessary for the system to indicate the direction of the pen from a magnetic signal null, or zero condition, location. To provide this coherent signal detection, the prior art, needing a reference signal for coherent detection, has resorted to the utilization of reference clock channels or complicated schemes for the generation or regeneration of clock reference signals to provide the necessary coherency.
The present invention eliminates the need for such clock channels and instead relies exclusively upon signal magnitude; the phase of the signals radiating from the cordless pen is not detected and not required and therefore it is not sensitive to phase shift errors. The elimination of reference clock channels or reference signal channels reduces the system of the present invention to the requirement of only a single channel. The single channel system is substantially less complex and provides the substantial advantages of reduced cost, power consumption, and increased applicability to environments requiring low power consumption such as portable or lap top computer systems. The present invention employs a particular grid pattern and a unique processing of the information of the non-coherent amplitude detected signals on the conductors to provide information that would normally be provided by signal phase detection to indicate pen direction relative to a particular grid conductor. The system of the present invention has thus eliminated the need for synchronous demodulators and replaced such demodulation techniques with simple amplitude detection. The simplicity afforded by this system yields a less expensive but reliable and light weight tablet that consumes minimum power and is particularly adaptable to applications requiring portability and mobility.
A variety of sensing or sampling techniques are utilized in the prior art to detect the presence of a pen or instrument in the proximity of a grid line and to determine the distance from the nearest grid or conductor to the instrument. These variety of techniques are intended obviously to present a precise location of the instrument on the tablet surface; these prior art techniques are frequently very complicated and incorporate complex tablet and grid configurations. In addition to accurately depicting the position of the instrument on the tablet, it is important that this position information reliably follow any change in the position of the instrument. If the instrument moves rapidly, such as the movement of a pen during handwriting, the positional routine followed by the prior art may experience difficulty making rapid adjustments to the fast moving pen or instrument. To compensate for the requirement to follow the fast
moving instrument, prior art techniques have imposed further complexities on the grid and detection system that increase cost, consume more power and decrease the quality of the position information (more "jitter"). For example, to follow a fast moving instrument, prior art techniques frequently have employed fixed or single formula routines and large line or element spacings in order to minimize the number of grid lines or elements that are scanned or addressed to resolve position and to increase the range over which each scan can resolve position. The present invention overcomes these prior art difficulties by providing both rapid coverage to accommodate fast moving instruments and accuracy routines for precision in the location of the instrument. The routines thus followed by the system of the present invention are adaptive and automatically accommodate a fast moving instrument while preserving the accuracy afforded by a slower, but more accurate, positioning routine.
When pens or instruments are radiating electromagnetic signals onto an arrangement of grid conductors, the signal pattern induced in the grid conductors disproportionally changes as the instrument approaches the edge of the tablet. This disproportionality results in an erroneous indication of pen position; however, this error may be reduced (or simply ignored) by increasing the size of the tablet and restricting the active area (that area of the tablet which may be used) to that portion of the tablet sufficiently distant from the edges of the grid. However, such prior art techniques render the use of such tablets in environments such as portable computer applications impractical since in such environments the tablet must be as small as possible and any prohibited area adjacent the edges of the tablet detracts from the available size of the tablet. The present invention utilizes a compensation technique to minimize edge error through the utilization of appropriately selecting a search routine and physically positioning grid conductors in a predetermined manner.
It is therefore an object of the present invention to provide a tablet for ascertaining the position of a radiating instrument wherein the system incorporates minimum components.
It is another object of the present invention to provide a graphic tablet system utilizing a cordless pen.
It is another object of the present invention to provide a graphic tablet system utilizing a pen for radiating electromagnetic energy onto the tablet wherein signals received by the tablet are amplitude detected only.
It is another object of the present invention to provide a single channel graphic tablet system utilizing a cordless pen.
It is still another object of the present invention to provide an improved graphic tablet system incorporating the utilization of a cordless pen and a simplified design using only a single channel and without clock reference signals.
It is another object of the present invention to provide a graphic tablet utilizing a cordless pen and non-coherent amplitude detection.
It is another object of the present invention to provide a graphic tablet incorporating a cordless pen wherein the pen down signal is generated and sensed exclusively through amplitude keying.
It is still another object of the present invention to provide a graphic tablet system utilizing decision directed routines for rapid acquisition of pen position and providing high accuracy or resolution.
It is still another object of the present invention to provide automatically implemented selective routines including routines that provide substantial linearity.
It is still another object of the present invention to provide a means for minimizing edge effects; that is, the deleterious effects caused by the proximity of the tablet edge when the pen is close thereto.
It is another object of the present invention to provide a graphic tablet system having high noise immunity.
It is still another object of the present invention to provide a graphic tablet system having very low power requirements and therefore particularly useable in portable and mobile computer applications.
It is still another object of the present invention to provide a graphic tablet system that employs decision directed routines that minimize the number of grid elements that need to be addressed.
Briefly, in accordance with the embodiment chosen for illustration, the present invention utilizes a tablet incorporating a plurality of orthogonally related grid loops, each of which is connectable to a AM detector through amplifying and bandpass stages. A single select frequency is utilized and the amplitude of the signal is compared to a predetermined threshold. The pen incorporates a current controlled oscillator radiating the single frequency and utilizes a means for generating either full power or reduced power. The signal from the pen is non-coherently detected and no clock signal is utilized.
The resulting signals on the grid loops are processed using a decision directed technique for selecting routines appropriate for the conditions at the time of signal sensing. The process of routine selection is automatic and accommodates rapid pen movement while maintaining accuracy.